Feature: Feedback when entering invalid credit card details
  In user testing we've seen a lot of people who made mistakes
  entering their credit card. We need to be as helpful as possible
  here to avoid losing users at this crucial stage of the
  transaction.
  
Background:
Given I have chosen some items to buy
And I am about to enter my credit card details

Scenario: Credit card number too short
When I enter a card number that's only 15 digits long
And all the other details are correct
And I submit the form
Then the form should be redisplayed
And I should see a message advising me of the correct number of digits

Scenario: Expiry date invalid
When I enter a card expiry date that's in the past
And all the other details are correct
And I submit the form
Then the form should be redisplayed
And I should see a message telling me the expiry date must be wrong